The answer is that the route table for the public subnet does not have a route to an internet gateway. For a NAT gateway to provide outbound internet access from a private subnet, it must itself be able to reach the internet; this requires the public subnet’s route table to include a default route (0.0.0.0/0) pointing to an internet gateway. Without this, the NAT gateway cannot forward traffic beyond the VPC, even though the private subnet’s route table correctly points to the NAT gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the layered routing dependencies in a VPC—a common trap is assuming a NAT gateway automatically has internet access simply because it resides in a public subnet. Remember that a public subnet is defined by having a route to an internet gateway, not just by its name. Memory tip: “NAT needs a path to the outside—if the public subnet lacks an IGW route, your private instance is stuck inside.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to access the internet to download software updates. The company has a NAT gateway in the public subnet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT gateway. However, the EC2 instance cannot access the internet. The network engineer verifies that the NAT gateway has an Elastic IP, the security group for the EC2 instance allows outbound HTTPS traffic, and the network ACL for the private subnet allows inbound and outbound ephemeral ports. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route table for the public subnet does not have a route to an internet gateway.
Option B is correct because the NAT gateway resides in the public subnet, and for it to route traffic to the internet, the public subnet's route table must have a default route pointing to an internet gateway. Without that, the NAT gateway cannot forward traffic. Option A is wrong because NAT gateways do not have security groups. Option C is wrong because the private subnet route table is correctly configured. Option D is wrong because the NAT gateway supports UDP as well.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The security group attached to the NAT gateway does not allow outbound traffic.
Why it's wrong here
NAT gateways do not have security groups; they are controlled by network ACLs.
✓
The route table for the public subnet does not have a route to an internet gateway.
Why this is correct
The public subnet must have a default route to an internet gateway for the NAT gateway to reach the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table for the public subnet does not have a route to an internet gateway. — Option B is correct because the NAT gateway resides in the public subnet, and for it to route traffic to the internet, the public subnet's route table must have a default route pointing to an internet gateway. Without that, the NAT gateway cannot forward traffic. Option A is wrong because NAT gateways do not have security groups. Option C is wrong because the private subnet route table is correctly configured. Option D is wrong because the NAT gateway supports UDP as well.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The instance is associated with a security group that allows outbound HTTPS (443) traffic. The route table for the private subnet has a default route pointing to a NAT Gateway in the public subnet. Which additional configuration is required to ensure the NAT Gateway can route the traffic?
hard
A.Configure the security group of the NAT Gateway to allow inbound traffic from the private subnet.
B.Add a route in the private subnet route table to the NAT Gateway's private IP.
✓ C.Add a route in the public subnet route table to the Internet Gateway.
D.Assign an Elastic IP address to the NAT Gateway.
Why C: The NAT Gateway must be in a public subnet with an Internet Gateway route. The security group of the NAT Gateway does not control traffic; it's a managed service. The NAT Gateway's Elastic IP is automatically assigned. The route table of the public subnet must have a default route to the Internet Gateway. The private subnet route is already correct.
Variation 2. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to initiate outbound connections to the internet. The company has a NAT gateway in the public subnet. The NAT gateway has an Elastic IP. The private subnet route table has a default route pointing to the NAT gateway. However, the EC2 instance cannot reach the internet. What is the most likely cause?
hard
A.The security group of the EC2 instance blocks outbound traffic.
B.The route table in the public subnet does not have a default route to the internet gateway.
C.The network ACL of the private subnet blocks inbound traffic from the NAT gateway.
✓ D.The NAT gateway's Elastic IP is not allowed by the destination's firewall.
Why D: Option D is correct because the NAT gateway's Elastic IP must be allowed by the on-premises firewall or the destination must allow traffic from that IP. Option A is incorrect because a default route exists. Option B is incorrect because security groups are stateful and allow outbound. Option C is incorrect because NACLs are stateless but inbound rule for ephemeral ports is needed; however, the issue is more likely the NAT gateway's public IP not being allowed.
Variation 3. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The company has a NAT gateway in the public subnet. The route table for the private subnet has a route 0.0.0.0/0 pointing to the NAT gateway. However, the EC2 instance cannot reach the internet. Which additional configuration is needed?
hard
A.Assign a public IP address to the EC2 instance.
B.Attach a security group to the NAT gateway allowing outbound traffic.
C.Add a route to the NAT gateway in the private subnet's route table for destination 0.0.0.0/0.
✓ D.Ensure the public subnet's route table has a route 0.0.0.0/0 pointing to an internet gateway.
Why D: The NAT gateway is in the public subnet, so the public subnet's route table must have a route to an internet gateway for the NAT gateway to function. Option A is wrong because the private subnet already has the correct route. Option C is wrong because security groups on the NAT gateway do not exist. Option D is wrong because the instance might not have a public IP, but it uses the NAT gateway for outbound traffic, so a public IP is not required.
Variation 4. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to download software patches from the internet. Which AWS service should be used to provide outbound internet access without allowing inbound traffic?
easy
A.Site-to-Site VPN connection
B.VPC endpoint
✓ C.NAT gateway
D.Internet gateway
Why C: Option A is correct because a NAT gateway allows outbound internet traffic from private subnets and blocks unsolicited inbound traffic. Option B is wrong because an internet gateway is for public subnets and allows inbound traffic. Option C is wrong because a VPC endpoint is for accessing AWS services privately, not the internet. Option D is wrong because a VPN connection is for hybrid connectivity, not internet access.
Variation 5. A company has a VPC with public and private subnets. An EC2 instance in a private subnet needs to download patches from the internet. The company has a NAT gateway in a public subnet. The EC2 instance can connect to other instances in the VPC but cannot reach the internet. What is the most likely cause?
medium
A.The security group of the EC2 instance blocks outbound HTTPS traffic.
B.The NAT gateway does not have an Elastic IP address.
C.The network ACL of the private subnet blocks inbound traffic from the NAT gateway.
✓ D.The route table in the private subnet does not have a default route to the NAT gateway.
Why D: Option C is correct because the route table associated with the private subnet must have a default route (0.0.0.0/0) pointing to the NAT gateway. Option A is incorrect because the NAT gateway itself does not require a public IP for outbound traffic (though it needs one for inbound). Option B is incorrect because security group rules are stateful and allow outbound traffic. Option D is incorrect because the NACL must allow outbound traffic, but the route is the primary issue.
Variation 6. A company has a VPC with a public subnet and a private subnet. An EC2 instance in the public subnet is configured as a NAT instance. The company wants to replace the NAT instance with a NAT gateway for better availability and maintenance. After creating a NAT gateway in the public subnet and updating the route table of the private subnet, traffic from the private subnet cannot reach the internet. What is the MOST likely cause?
medium
A.The public subnet's route table still points to the NAT instance for internet traffic.
B.The security group attached to the NAT gateway is blocking outbound traffic.
C.The private subnet's route table has a route for 0.0.0.0/0 pointing to the NAT instance instead of the NAT gateway.
✓ D.The NAT gateway does not have an Elastic IP address associated with it.
Why D: The NAT gateway must have an Elastic IP to work. Option A is wrong because the route table should point to the NAT gateway. Option B is wrong because the private subnet's route table destination for 0.0.0.0/0 should be the NAT gateway. Option D is wrong because security groups on the NAT gateway are not applicable (NAT gateway is managed).
Variation 7. A company has a VPC with a public subnet and a private subnet. They launch an EC2 instance in the private subnet with a private IP only. The instance needs to download patches from the internet. Which configuration is required?
medium
A.Launch a NAT instance in the private subnet and configure source/destination check.
B.Create a VPC endpoint for the patch service.
✓ C.Create a NAT gateway in the public subnet and add a default route to the NAT gateway in the private subnet's route table.
D.Attach an internet gateway to the VPC and add a default route to it in the private subnet's route table.
Why C: Option B is correct because a NAT gateway in the public subnet allows instances in private subnets to initiate outbound traffic to the internet. Option A is incorrect because an internet gateway alone does not allow private instances to access the internet. Option C is incorrect because a NAT instance requires manual configuration. Option D is incorrect because a VPC endpoint is for specific AWS services, not general internet access.
Variation 8. A company has a VPC with public and private subnets. The private subnets need to access the internet for software updates. The company wants to ensure that traffic can only go out to the internet and not be initiated from the internet. Which configuration should be used?
medium
A.Attach an Internet Gateway to the private subnets
✓ B.Deploy a NAT Gateway in a public subnet and update the private subnet route table
C.Set up a VPN connection to an on-premises network
D.Create a VPC endpoint for S3
Why B: Option B is correct because a NAT Gateway in a public subnet allows private instances to initiate outbound traffic to the internet while blocking inbound traffic. Option A is incorrect because an Internet Gateway allows both inbound and outbound traffic. Option C is incorrect because a VPC endpoint is for accessing AWS services, not the internet. Option D is incorrect because a VPN connection is for private connectivity to on-premises.
Last reviewed: Jun 20, 2026
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